The invention relates to radio networks, that are networks in which units wirelessly exchange information by way of radio signals. In particular, radio networks in which the air interface applies frequency hopping to spread the signal over a wide spectrum are considered. The problem addressed is the multiple access of different units on a common, frequency hopping channel.
The system considered is based on a frequency hopping (FH) system, different aspects of which are described in U.S. patent application Ser. Nos. 08/685,069; 08/932,911; and 08/932,244; as well as U.S. Provisional Application No. 60/109,692, filed on Nov. 24, 1998 in the name of J. Haartsen which are all hereby incorporated herein by reference. In this system, a channel is defined as a frequency hop sequence which is a pseudo-random number (PN) sequence determined by the identity of one of the units participating on the channel, called the master. The phase in the sequence is determined by a master clock associated with the master. As the master clock progresses, the channel hops from radio frequency (RF) hop frequency to RF hop frequency at the clock rate. All other units participating on the channel, called slaves, are synchronized to the FH scheme by using the same FH sequence and same clock as used by the master. The channel shared between the master and the one or more slaves is called a piconet.
At connection setup, the master parameters that are required to maintain FH synchronization are transferred from the master to the slave. A strict Time Division Duplex (TDD) scheme is adhered to: time slots (xe2x80x9cslotsxe2x80x9d) in which traffic is transferred from master to slave and slots in which traffic is transferred from slave to master, alternate at the hopping rate. Preferably, a high hopping rate is used in order to obtain immunity against interferers that share the spectrum. A high hopping rate results in short slots and small packets.
The master controls the access on the channel. A distributed access method, like carrier-sense multiple access, is not useable due to the fast hopping of the channel; the dwell time on a RF hop frequency is too short to carry out an effective contention-based access scheme. On the other hand, reserved access schemes like TDMA are not suitable for packet-switched data connections. Therefore, a polling scheme is used which is entirely controlled by the master of the piconet. At any moment in time, a master may select any of the slaves participating on the channel to send data to in the master-to-slave slot. However, only the slave addressed by the master in this master-to-slave slot may respond in the succeeding slave-to-master slot.
In this scheme, the master selects a slave in the master-to-slave slot to send data to and from which it can receive data. As a result, collisions between slaves that want to send information to the master at the same time are prevented. When the master sends information to slave X, this implicitly means that slave X may respond in the next slave-to-master slot. The slave is implicitly polled by the master. If the master has no data to send, it may send a specific xe2x80x98pollxe2x80x99 packet to give the slave a chance to respond. A poll packet is a very short packet carrying no data.
The addressing scheme in the system is carried out as follows. Each unit has a unique identity which is, for example, derived from the 48-bit IEEE 802 addressing space. The identity of the master is used to form the FH sequence used by the channel in the piconet. Each packet is preceded by a preamble which is also derived from the master identity. This preamble is used by all the units participating in the piconet to identify whether there is a packet present in the slot, and if so whether the packet belongs to this piconet. Since many uncoordinated frequency hopping piconets may be co-located, occasionally they may happen to land on the same hop frequency. The preamble prevents the users in one piconet from accepting packets belonging to another piconet. The master address therefore identifies the piconet (or channel) and can be regarded as a channel identifier.
To distinguish between the different participants on the piconet, a short length Medium Access Control (MAC) address is used which is temporarily allocated by the master to the slave when the slave is connected to the piconet. The MAC address is located in the header of the packet. The master uses the proper MAC address to address a slave. The size of the MAC address is preferably small in order to minimize the overhead in the packet header. As was mentioned before, the system preferably uses a fast hopping rate. As a result, the packet can only be short and the amount of overhead (including the MAC address) must be minimized. However, the use of only a short-length MAC address limits the number of slaves that can simultaneously participate on the channel.
Slaves that do not have to exchange a great deal of information can be placed in a low power mode called HOLD. When the slave is in the HOLD mode, it does not participate on the channel. It neither transmits nor receives data, but it does keep its clock running (so that it remains synchronized to the FH channel), and it retains its MAC address. At the conclusion of a HOLD interval (the duration of which is agreed upon by both the master and the slave prior to entering the HOLD mode), the slave leaves the HOLD mode and participates on the channel as before.
Units that want to remain locked to the channel can enter the HOLD mode to save power consumption. However, since they keep their MAC addresses, units that rarely participate on the channel deny access to the channel to other units since the MAC address space is limited. This inefficient use of the MAC addresses is more of a problem in those described FH systems in which the MAC address is short (to minimize overhead), resulting in only a few slaves being able to participate on the channel.
It is therefore an object of the present invention to provide techniques for keeping units synchronized to the channel in a piconet without requiring them to retain their MAC addresses.
The foregoing and other objects are achieved in apparatuses and methods of operating a system comprising a wireless master unit and one or more wireless slave units, wherein each of the one or more wireless slave units has a unique identifier. In accordance with one aspect of the invention, a wireless slave unit may be in a so-called PARK mode, in which it is not associated with a temporary address (e.g., a MAC address described in the BACKGROUND section). In order to page a parked wireless slave unit, a paging beacon packet is broadcast to, and received in, each of the one or more wireless slave units at fixed intervals during a master-to-slave time slot. Each wireless slave unit, determines whether the received paging beacon packet includes the unique identifier belonging to the wireless slave unit. If it does, then the wireless slave unit retrieves a temporary address from the paging beacon packet, and transmits a response to the wireless master unit during a subsequent slave-to-master time slot.
In another aspect of the invention, the wireless slave unit can determine whether a subsequent traffic packet from the wireless master unit includes the temporary address and, if so, respond by transmitting a response to the wireless master unit during another subsequent slave-to-master time slot.
In yet another aspect of the invention, the paging beacon packet is a type of beacon packet, wherein beacon packets have a header portion that includes a predefined temporary address that is never assigned to any of the one or more wireless slave units in the system.
In still another aspect of the invention, parked wireless slave units are offered an opportunity to request access to the piconet. This is accomplished by defining a series of time slots comprising alternating occurrences of a master-to-slave time slot and a slave-to-master time slot, wherein each of the slave-to-master time slots comprises a plurality of slave-to-master sub-slots. Depending on the embodiment, the number of sub-slots per slave-to-master time slot may be any integer greater than or equal to 1. Furthermore, a unique response number is allocated to each of the one or more wireless slave units. A polling beacon packet is broadcast by the master unit to each of the one or more wireless slave units at fixed intervals during a master-to-slave time slot. Receipt of the polling beacon packet by a wireless unit indicates an opportunity to request access to the piconet. Accordingly, if a wireless unit desires to access the piconet, it transmits a packet to the wireless master unit during a slave-to-master sub-slot that occurs N slave-to-master sub-slots after the polling beacon packet, wherein N is a function of the unique response number of the at least one or more wireless slave units.
In yet another aspect of the invention, the master unit is not required to give each of the wireless units an opportunity to respond to the polling beacon packet. To accommodate this possibility, a slave unit detects whether any master activity occurred in the master-to-slave time slot immediately preceding the slave-to-master sub-slot that occurs N slave-to master sub-slots after the polling beacon packet, and if so, transmits the packet to the wireless master unit only if no master activity was detected in the master-to-slave time slot immediately preceding the slave-to-master sub-slot that occurs N slave-to-master sub-slots after the polling beacon packet.
In still another aspect of the invention the wireless master unit receives the response packet from the at least one of the one or more wireless slave units, and determines which of the one or more wireless slave units transmitted the packet by determining which slave-to-master sub-slot the packet was received in, relative to the master-to-slave time slot during which the polling beacon packet was broadcast.